Lithium disilicate glass ceramics are known. EP-B-536 479 describes self-glazed lithium disilicate glass ceramic articles which are not, however, intended for dental applications. The glass ceramics are free of La.sub.2 O.sub.3 and are formed in the usual manner by melting suitable starting materials, pouring the melt into molds to produce articles, and subsequently heat treating the articles.
Lithium silicate glass ceramics are also disclosed in EP-B-536 572. They are given structure and color by the dispersion of a finely divided colored glass onto their surface, and they are used as lining units for building purposes. They are manufactured in a conventional manner in that suitable starting materials are melted, the melt is molded to a desired body, and the body is heat-treated together with dispersed colored glass. La.sub.2 O.sub.3 is not, however, contained in the glass ceramic.
Glass ceramics based on SiO.sub.2 and Li.sub.2 O which contain large quantities of poisonous arsenic oxide are known from DE-C-1 421 886.
Moreover, the use of lithium disilicate glass ceramics in dental technology is also known, but these glass ceramics do not contain La.sub.2 O.sub.3 or MgO. Further, only conventional methods are used to process these glass ceramics to dental products, where a heat treatment is carried out to precipitate crystals only on homogeneous bodies, namely monoliths formed from a glass melt, such as small glass block or slabs. Conventional methods of this kind, however, only allow volume crystallization to take place, not surface crystallization.
Examples of such glass ceramics and conventional methods for the production thereof are described in following documents.
A lithium disilicate glass ceramic with high strength suitable for the preparation of dental crowns and bridges is described in U.S. Pat. No. 4,515,634.
A high-strength lithium disilicate glass ceramic is also described in U.S. Pat. No. 4,189,325 wherein said glass ceramic necessarily contains CaO to improve the flow and also platinum and niobium oxide to produce very fine and uniform crystals.
Glass ceramics containing lithium oxide and silicon oxide for the preparation of dental prostheses, which contain very large quantities of MgO, are described in FR-A-2 655 264.
Finally, U.S. Pat. No. 5,507,981 and WO-A-95/32678 describe lithium disilicate glass ceramics which may be further processed to formed dental products by special methods, wherein pressing in the viscous, flowable state at elevated temperatures to the desired dental product takes place. No further details are given regarding the production of the slabs or buttons used during this process. A conventional method is also used to produce the glass ceramic, in that homogeneous glass bodies, such as slabs, for example, are heat-treated. A disadvantage of these methods is that they are very labor intensive for a dental technician, due to the use of a special heat-pressure deformable crucible. Moreover, the glass ceramic materials are heated to such an extent that crystals are no longer present in the molten material since the viscosity would otherwise be to high for pressing to the desired dental product. Consequently, the product processed is glass, not a glass ceramic.
The above-noted lithium disilicate glass ceramics have shortcomings, particularly when they are to be processed further in the plastic state to form a shaped dental product. Their viscosity is not ideally adjusted for such processing, so a controlled flow is not possible and the reaction with the investment material is undesirably high. Moreover, conventional glass ceramics have only poor dimensional stability on heating, so that dental restorations produced from them may be provided with a sintered-on glass or glass ceramic layer only with deformation. Finally, conventional lithium disilicate glass ceramics also frequently lack the necessary chemical stability for use as dental material, which is permanently being flushed with fluids of various kinds in the oral cavity.